We studied erythrocyte sodium-lithium countertransport in 33 patients with Type 1 (insulin-dependent) diabetes mellitus with diabetic nephropathy, 18 patients with Type 1 diabetes without diabetic nephropathy and in 42 non-diabetic patients with various other renal diseases. No significant differences were found in sodium-lithium countertransport between these three groups (median (range) 322 (162-676) vs 321 (189-627) vs 300 (142-655) mumol.l cells-1.h-1). We conclude, that sodium-lithium countertransport cannot be used as a marker for diabetic nephropathy.